Many people experience difficulties with their vision due to a number of possible conditions. The most common vision problem is a condition known as myopia or nearsightedness. Myopia is a common condition where an eye cannot focus on far-away objects because the cornea of the eye is curved too steeply (i.e., where the radius of curvature of the cornea is smaller than normal) to provide adequate focusing at the retina of the eye. Another condition is known as hyperopia or farsightedness. With hyperopia, the eye cannot focus on both far and near objects because the curvature of the cornea of the eye is too flat (i.e., where the radius of curvature of the cornea is larger than normal) to provide adequate focusing at the retina of the eye. Hyperopia is common among young children. Severe hyperopia will induce lazy eye or amblyopia in childhood. Another common problem is astigmatism, where unequal curvature of one or more refractive surfaces of the cornea prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.
Presbyopia is the most common vision problem in adults 40 years and older. It does not matter whether they are emmetropic, myopic or hyperopic in distance vision, the middle-aged population over 40 years old will begin to experience difficulty in focusing on close objects, due to the loss of flexibility of the eye's crystalline lens. Presbyopia may occur and complicate other refractive problems such as hyperopia, myopia or astigmatism.
Presbyopia is a condition for which no entirely suitable permanent treatment has been developed. The most common conventional way is to wear spectacle glasses. The spectacle glasses can comprise two pairs of single vision glasses, one pair for near vision and another for far. The glasses can also be incorporated in a single pair of multi-focal glasses that has two or more focal points in different zones of the glasses by alternating the visual or optical axis between far and near zones. One approach to correcting presbyopia is through laser surgical reshaping of the cornea. However, such surgical procedures have not been entirely safe and there have been less favorable vision outcomes for presbyopia surgery than for myopia surgery. Presbyopia can also be corrected by the implantation of intraocular lenses (IOL) with multifocal designs to substitute for a subject's original crystalline lens for patients receiving cataract surgery.
Another way for correcting presbyopia is to wear contact lenses with multi-focal designs. There are two major types of multi-focal contact lenses for correcting presbyopia, the translating and simultaneous vision designs. The former type (the translating multi-focal contact lens) has at least two separate areas or zones for far and near vision respectively. The reading segment has to be positioned appropriately for effective reading and yet not interfere with distant vision. The user usually has to read with a posture of head tilting or downward gazing to appreciate the translating bifocal.
Simultaneous vision contact lenses can be a bifocal or multifocal in design. In either design, the center of the lens can be the distance (CD or Center distance) or near (CN or center near) refractive power. Multi-focal designs generally have at least two distinct zones of two different powers. Most of the multi-focal contact lenses are aspheric and have a gradient power changing with a certain eccentricity value from the lens center outward. The simultaneous multi-focal lens forms an image mixing both distant and near focus with a variety of focused and defocused portions of the image. The simultaneous vision lens, though more convenient for less presbyopic patients with lower ADD, is far less satisfactory in more advanced presbyopia.
Another approach to treating presbyopia is to alter the corneal shape by wearing contact lenses which are designed to continually exert pressure on selected locations of the cornea to gradually force or mold the cornea into the desired normal corneal curvature. This method of treatment is commonly referred to as orthokeratology (referred to herein as “ortho-k”). Treating presbyopia using ortho-k lenses has been disclosed in the U.S. Pat. Nos. 6,652,095 and 7,070,275 to Tung, for example.
Notwithstanding the improvements provided by modern spectacles, contact lenses, intraocular ocular lenses, refractive surgery and orthokeratology for presbyopia, there remains a need for optical devices that achieve better multi-focal correction for presbyopia.